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CLEANROOMS AND CLEAN AIR

Cleanrooms are highly controlled environments where the air quality is monitored to ensure the extreme standards of cleanliness required for surgical units and for hospital pharmacies.

A cleanroom is as a specially designed room in which the concentration of airborne contamination is controlled, and which is constructed and used in a manner to minimise the introduction, generation and retention of particles inside the room and in which other important parameters, such as pressure, temperature and humidity are controlled1. In order to supply cleanrooms with clean, high fresh air rates, extensive filtering, temperature and humidity control are required.

The supply of clean air to cleanrooms, and the control of the parameters of temperature, humidity and pressure, is through HVAC (Heating Ventilation and Air Conditioning) systems (sometimes called air handling systems). A fundamental part of the HVAC system are the specially designed filters, termed HEPA (High Efficiency Particulate Air) filters, which are designed to reduce the number of microorganisms and other particles down to an appropriate level of cleanliness.

This article examines how air handling systems and HEPA filters work and discusses some of the important tests required in order to verify systems to demonstrate that they continue to work efficiently and keep cleanrooms clean.

Contamination controland airflowBefore examining air handling systems and HEPA filters it is important to review cleanroom contamination. Contamination control is the primary consideration in cleanroom design2; however, the relationships between contamination control and airflow are not well understood. Supplying cleanrooms with clean air, through HEPA filters, means that most cleanrooms are supplied with ‘clean air’. Contaminants in cleanrooms, such as particles or microorganisms, are primarily introduced to cleanrooms by people, although processes in cleanrooms may also introduce contamination3. With contamination present in cleanrooms, the supply of clean air is necessary in order to remove contamination through the replacement of the room air volume with clean air.

There are three ways in which particles could get into a cleanroom:

• From outside air – even in the countryside, away from factories, air contains approximately 108 particles of 0.5µm size and greater per cubic metre

• From the wear and tear of the HVAC system

• Generated by personnel working within the cleanroom

Although air is a source of contamination, however, microorganisms do not grow and increase in numbers in air and most microorganisms find it hard to survive in air because the environment desiccates them, and also because of the exposure to ultraviolet radiation. Thus the risk to cleanrooms is from those microorganisms in the air settling onto critical surfaces.

Although air is a contamination source, air is also the answer to many contamination problems. There are four principles which apply to the control of airborne contamination in cleanrooms. These are:

• Filtration: cleanrooms need to be designed so that most of the contamination in the air is filtered out

• Dilution: cleanrooms need to be supplied with a sufficient volume of fresh air at regular intervals so that any contamination generated by people working in the room is at first diluted and then removed from the room. This is achieved by having a set number of air changes per hour. The minimum requirement is normally 20 air changes per hour; that is, the

room air volume is replaced every three minutes

The importance of air handling and HEPA filtration

Dr Tim Sandle, UK Bio Products Laboratory

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• Directional Air Flow: for ultra-clean activities, undertaken in unidirectional airflow cabinets operating at EU and WHO Grade A (ISO class 5), the air needs to move in a straight direction so that any contamination generated within the area is removed. This is achieved by having the air enter at a high velocity – normally at 0.45 metres per second ±20%

• Air Movement: the air within cleanrooms needs to keep moving so that any contamination remains suspended in the air rather than being allowed to settle onto surfaces. This is achieved by having unidirectional or turbulent airflow

Heating ventilation andair conditioningThe key design consideration for the cleanroom is the HVAC (Heating Ventilation and Air Conditioning) system. HVAC systems were first designed by mechanical engineers for the electronics industry and for the manufacture of aircraft parts in World War II. Post war, the application of HVAC to provide clean air was orientated towards the healthcare and pharmaceutical sectors. The functions of HVAC are:

• To heat the air

• To supply ventilation – the process of replacing air in a room in order to remove heat, dust, and airborne bacteria

• To condition air – this involves the control of humidity (dehumidification) or the removal of heat to cool the air to required level

• Maintain room pressure (delta P) – areas that must remain ‘cleaner’ than surrounding areas must be kept under a ‘positive’ pressurisation (a concept examined below)

• In conjunction with HEPA filters, to control airborne particulates and microorganisms

For healthcare cleanrooms airflow and air changes must be controlled, and the temperature and humidity of the air

maintained at appropriate levels. In addition, the air must be filtered and for this the cleanroom requires appropriate HEPA (high efficiency particulate air) systems.

The HVAC system operates on the basis of mechanical ventilation. An example of the air supply is illustrated in Figure 1.

Air handling units consist of filters, coils and fans in a metal casing, with an insulation liner applied to the inside of the casing. For healthcare applications the unit casing must be a double skin sandwich of metal with insulation between the metal sheets to provide a smooth, cleanable interior surface that does not foster the growth of microorganisms. The two most important elements are the fans, which control the air speed, and the HEPA filters.

High efficiency particulate air filtersa) How HEPA filters work

The HEPA filter is a fundamental part of the HVAC system. The filters are designed to control the number of particles entering a clean area by filtration. HEPA filters function through a combination of three important aspects. First, there are one or more outer filters that work like sieves to stop the larger particles of dirt, dust, and hair. Inside those filters, there is a concertina – a mat of very dense fibres arranged in a random order. The fibres are normally made from fibreglass. The function of the fibres is to trap smaller particles. The inner part of the HEPA filter catches particles as they pass through in the moving air stream, through three mechanisms:

• Interception. For this, particles following a line of flow in the air stream come within one radius of a fibre and adhere to it

• Impaction. Here, larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher air flow velocity

• Diffusion. Here, smaller particles are impeded by colliding with gas molecules which impedes the movement

of the particles

These concepts are illustrated in Figure 2.

Filter class

Overall value efficiency (%)

Overall Value Penetration (%)

Leak test efficiency (%)

Leak test penetration (%)

H 10 85 15 - -

H 11 95 5 - -

H 12 99.5 0.5 - -

H 13 99.95 0.05 99.75 0.25

H 14 99.995 0.005 99.975 0.025

U 15 99.9995 0.0005 99.9975 0.0025

U 16 99.99995 0.00005 99.99975 0.00025

U 17 99.999995 0.000005 99.9999 0.0001

Table 1 - Classification of air filters based on standard EN 18224

Figure 1 - Typical air ventilation supply

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b) Standards for HEPA filters

There are different grades of HEPA filters based on their efficiency ratings. These are shown in Table 1.

There is a second type of filter called a ULPA (Ultra Low Penetration Air) filter. These filters are capable of a far greater particle reduction than HEPA filters. They are expensive to operate, however, and tend to be used more often in the manufacture of electronic circuits. The type of filter purchased depends upon the requirement of the facility. Most healthcare facilities use H13 or H14 HEPA filters.

Arguably the most important parameter of a HEPA filter is the efficiency value. This determines how many particles the HEPA filter will theoretically let pass through into a cleanroom. This is based on the theoretical assumption that if 1,000 particles of a size of 0.3 µm size are challenged to the filter then only x number will pass through. An H13 rated filter, for example, is capable of filtering out 99.97% particles of 0.3 µm or larger.

c) Positioning HEPA filters

Most HEPA filters are ceiling mounted, which allows the HEPA filters to supply air with sufficient velocity and volume to unidirectional sweep over the critical areas. Sometimes HEPA filters are located lower down within walls, although in general this makes the filters less efficient at distributing the airflow.

See Figure 3

d) Modification of HEPA filters

For some healthcare applications greater assurance that pathogens have been removed from the air supply is required. For this, ultraviolet (UV) lighting is used. The total power of the UV lamp is a product to the power of the light source multiplied by the exposure time. A typical specification for a UV light is a 254 nanometer wavelength UV light operating at 6,500 mW /cm2.

Function of air handlingsystems and HEPA filtersfor contamination controlThe key function of HEPA filters is air filtration. Filtration removes particles and microorganisms. Although HEPA filters are efficient at removing particles, they need to be protected from blockage by pre-filters, otherwise the life span of the HEPA filter would be relatively short. Pre-filters remove up to about 90% of particles from air5.

To illustrate the importance of pre-filters, if air contains about 3 x 108 particles per m3, and there is one pre-filter and one HEPA filter, the pre-filter removes a sufficient number of particles to leave about 3 x 107 per m3 as a challenge to the HEPA filter. The terminal HEPA filter will leave about 103 per m3 to enter the cleanroom. This is a relatively low number and is within the limits for EU and WHO GMP Grade A (ISO class 5) and Grade B (ISO class 7).

In fact, most air handling systems recirculate up to 80% of the air supplied to cleanrooms. Therefore the initial challenge to the HEPA filters is probably only about 106 particles per m3. So in practice there is normally no more than 3 x 102 particles per m3 supplied to pharmaceutical cleanrooms. This level is even further within the required limits.

Air handing systems and HEPA filters also reduce contamination through air movement. The cleanroom air flow pattern should be designed to provide sufficient air to constantly cover all critical areas where risk is high – particularly where the patient or product is exposed. The physical bathing of the critical areas with the filtered air acts as a shield to help prevent contaminates from reaching the exposed products.

Ceiling mounted HEPA filters can supply air with sufficient velocity and volume to unidirectional sweep over the critical areas. It is typical for air handling systems to provide sufficient air volume to allow multiple complete air changes in the cleanroom. This flushes the room with filtered air and functions to continually remove potential contaminants that may enter or accumulate in the room during normal use. The number of air changes required for cleanrooms where a product is exposed is often sited to a minimum standard not less than 20 air changes per hour, but in practice it is common to see cleanrooms designed to provide a much higher number of air changes.

Interception

Figure 2 - Illustration of the concepts of interception, impaction and diffusion in relation to HEPA filters

Impaction

Diffusion

Figure 3 - HEPA filter

Seals

Gaskets

HEPA Filter

Integrity test probe

False ceiling

Inlet duct

Structural slab

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Most cleanrooms are of the turbulent air flow type. Here, air is

driven in through grilles and ducts at ceiling height and

removed through low level ducts. While the air is in the room

its initial supply velocity is sufficient to keep it in constant

turbulence, which prevents particles and microorganisms from

settling settle out. This is an ideal because dead air can occur

beneath objects such as tables. It is important to know where

dead areas occur – these can be shown through airflow

visualisation studies using smoke. If they cannot be avoided

then monitoring should be targeted at these locations.

“particles and microorganisms cannot ‘swim upstream’ against a directional air flow”

The amount of air supplied and exhausted in a cleanroom

compared to that supplied to the adjacent rooms is also used

to create a pressure differential cascade between the

cleanroom and the surrounding rooms. This is an essential

function to control contamination entering from room to room

together with materials and personnel. The pressure cascade should be designed to have a higher pressure where the more

critical operations are being performed. Values of 10-15 Pascal

between rooms of different grades are typical.

The way in which the ‘dirty’ air is prevented from entering the

cleanroom is by ensuring a very high rate of air supply to the

cleanroom, thus keeping it at a higher pressure than its

surroundings. If there is contact with ‘outside’ air, any

mixing of the two types of air takes place outside the

cleanroom because the direction of air flow is from the clean

to the dirty area.

This directional air flow is measured and monitored through

pressure differentials. Thus air will always move from an area

where it is at a high pressure to one where there is a low

pressure –this is due to a Law of Physics. Particles and

microorganisms cannot ‘swim upstream’ against a directional

air flow.

Routine monitoringIn order to ensure that control measures are being maintained

and to examine the status of the cleanroom, the air handling

system must be monitored closely and maintained regularly.

This includes routine monitoring of particulates using optical

particle counters; viable (microbial) environmental

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monitoring using methods which employ a microbiological agar culture medium; assessing differential pressures; and examining the parameters of temperature and relative humidity6.

Annual verificationIn order to ensure that HVAC systems continue to work effectively, annual testing the HEPA filters (e.g. integrity-leak tests, air velocity) and air flow patterns via smoke tests are essential parts of the regular maintenance of the air handling system. Other tests include: assessing the air stream velocity under each filter panel; taking airflow measurements at the supply, return and exhaust outlets; particulate testing of the cleanroom to show that the system can deliver air of appropriate quality; measuring pressure differentials between the cleanroom and adjacent areas; assessing the pressure drop across the final filters and recording room temperatures and relative humidity.

“regular performance checking and annual calibration is required in order to maintain control”

The main failure risk is from leaks, where the concern is with pinhole leaks in the HEPA filter media, across sealants and frame gaskets, supporting frame and wall. A leak would allow a higher concentration of particles to enter the cleanroom. To check their effectiveness they are checked for leaks by challenging the filters with a particle generating substance (an aerosol challenge, for example Di-octyl phtalate [DOP] or Shell Ondino mineral oil) and measuring the efficiency of the filter.

Leakage is assessed using an aerosol photometer. The aerosol photometer has to be fixed in a way to be able to measure the concentration of aerosol in front of and behind the filter. A photometer assesses the HEPA filter by taking a sample of the air at the filter downstream. When particles pass through a special light within the photometer, they reflect the light and this light passes through lenses and through a photomultiplier where the light is converted into electric signal. The result is the higher the number of particles the stronger the signal. The photometers usually measure the concentration of aerosol from 0.0001 µg/l to 100 µg/l – the instrument functions by measuring the weight of particles rather than counting the actual number.

SummaryThis article has outlined the importance of the supply of clean air to cleanrooms and has discussed the importance of HEPA filters in achieving this clean air supply.

For users of cleanrooms it is important to specify the types of air handling systems and HEPA filters required and to ensure that the cleanroom is designed in an appropriate way. Once established, regular performance checking and annual calibration is required in order to maintain control.

References1. ISO 14644-1: Cleanrooms and associated controlled environments - Part 1:

Classification of air cleanliness. International Organization for Standardization ISO, Geneva (May 1999)

2. Ljungqvist B., Reinmüller Berit: Cleanroom design - Minimizing contamination through proper design. Interpharm Press, Buffalo Grove IL/USA (1997).

3. Austin Ph.R.: People generated contamination. Journal of the American Association of Contamination Control 5 (1966) no. 1.

4. EN 1822: High efficiency particulate air filters (EPA, HEPA and ULPA): Part 1 - Classification, performance testing and marking; Part 2 - Aerosol production, measuring equipment, particle counting statistics; Part 3 - Testing flat sheet filter media; Part 4 - Determining leakage of filter elements (scan method); Part 5 - Determining the efficiency of filter elements. Idem, ibid. (Nov. 2009).

5. Jarmey-Swan, C. Filtration. [ed.] Norman Hodges and Geoff Hanlon. Industrial Pharmaceutical Microbiology Standards & Controls. Haslemere : Euromed Communications, 2008, S7.

6. Sandle, T. (2011): ‘Environmental Monitoring’ in Saghee, M.R., Sandle, T. and Tidswell, E.C. (Eds.) (2011): Microbiology and Sterility Assurance in Pharmaceuticals and Medical Devices, New Delhi: Business Horizons, pp293-326

AuthorDr Tim Sandle is a chartered biologist and has worked for various healthcare and pharmaceutical organisations. Dr Sandle is an honorary consultant with the University of Manchester. In addition, he serves on several international committees relating to pharmaceutical microbiology and cleanroom contamination control.

Dr Sandle has written more than 100 book chapters, peer reviewed papers and technical articles relating to microbiology. He is co-editor of the comprehensive book ‘Microbiology and Sterility Assurance in Pharmaceuticals and Medical Devices’. Dr. Sandle has also delivered papers to more than 40 international conferences.

Dr Sandle operates a microbiology and healthcare blog http://www.pharmig.blogspot.com

He can be contacted at: timsandle@btinternet.com